The present invention relates to a color-image forming apparatus equipped with a photosensitive drum and a transfer belt that forms an image by overlapping several pieces of image information through electro-photography and other technologies.
In the prior-art color-image forming apparatus applying electro-photography, an image has been typically formed through the procedures below:
i) a charger charges a photosensitive drum serving as an image-carrier;
ii) with acceptance of laser radiation according to image information, the photosensitive drum forms an electrostatic latent image thereon;
iii) a developing unit develops the electrostatic latent image into a visible toner image; then
iv) the visible toner image is transferred onto a sheet of paper or other sheet-type materials.
Responding to the needs for color image, many kinds of tandem type color-image forming apparatuses have been developed so far.
A typical tandem type apparatus has plural image-carriersxe2x80x94each carrier is responsible for carrying cyan-, magenta-, yellow-, and preferably black-images. The individual four images are formed on their respective carriers in the series of image-forming processes described above. All of the separately carried images are overlapped at a proper position of each carrier and transferred onto a recording material to form a full-color image.
In another tandem type color-image forming apparatus, the toner images formed on the respective image-carriers are temporarily transferred onto an inter-stage transfer member one upon another. After that, the overlapped full-color toner image is transferred onto a recording material in one operation.
Such a tandem type apparatus contributes to high-speed image forming due to its structure in which each color image has its specific image-forming section.
FIG. 20 shows a schematic diagram of the prior-art color-image forming apparatus. In the apparatus shown in FIG. 20, image-forming units 41, 42, 43, and 44 form toner images colored in black (K), cyan (C), magenta (M), and yellow (Y), respectively. According to an image signal from an exposure unit 45, photosensitive drums 41a, 42a, 43a, and 44a disposed in image-forming units 41, 42, 43, and 44, respectively, form each electrostatic latent image thereon. Inter-stage transfer belt 46 has a closed-loop structure. Transfer unit 47 transfers the respective toner images formed on drums 41a, 42a, 43a, and 44a onto belt 46. Fixing unit 48 finally transfers the toner image from belt 46 to sheet P fed from paper cassette 49 and fixes the image into place.
Here will be described the inner structure of image-forming units 41 through 44 in some detail. In image-forming unit 41 for black (K), for example, a laser beam from exposure unit 45 creates an electrostatic latent image over the peripheral surface of photosensitive drum 41a. Developing roller 41c applies toner onto the latent image formed on drum 41a to obtain a visible image. Cleaner 41d cleans out the residual toner on drum 41a after the toner image has been transferred onto belt 46. The procedure described above is performed in other units 42 through 44.
In such structured color-image forming apparatus, here will be described, for example, how the latent image carrying black component is formed on the drum.
Firstly, Charger 41b evenly charges over photosensitive drum 41a responsible for black. Then, according to image information from the host computer (not shown), exposure unit 45 applies laser light onto drum 41a to create a latent image. A thin layer of toner on developing roller 41c allows the latent image to be visible as a black-toner image. Having contact with drum 41a, inter-stage transfer belt 46 travels in the direction indicated by the arrow A shown in FIG. 20. The visible black-toner image is transferred onto belt 46 through the application of transferring pressure from the inside of belt 46 to transfer unit 47.
In the meantime, the latent image of the cyan component is created and then developed into a visible cyan-toner image with the help of cyan-toner layered on developing roller 42c. The cyan-toner image is transferred to belt 46 where the transfer process for the black-toner image has just been provided, thereby the two images are overlapped with each other.
In the same manner, the magenta- and the yellow-toner images are formed and overlapped with one after another. Thus, belt 46 carries the four-color overlapped image thereon.
The full-color toner image on belt 46 is finally transferred by transfer roller 50 onto sheet P coming from paper cassette 49. After that, sheet P travels through fixing unit 48 to have the toner image fixed thereon, and goes out in the direction B shown in FIG. 20.
After the toner image has been transferred onto sheet P, the residual toner on belt 46 is cleared out by belt cleaner 51.
According to the prior-art apparatus, however, its structurexe2x80x94the photosensitive drum contacting with the inter-stage transfer belt at all timesxe2x80x94can raise a problem. If there is a difference in speed between the rotation of the drum and the running of the transfer belt, damage or wear can occur on the surfaces rubbing against each other, which may result in degradation in image quality or loss of life.
A suggestion that the drive timing of the drum should agree with that of the belt may be a remedy for the problem described above. However, initiating the operation of the two units with exact same drive-timing is practically impossible due to time-lags of the driving systemsxe2x80x94delay in response of a motor, gears, and an actuatorxe2x80x94of the drum and the belt. Therefore, minute abrasion will persist in such a situation.
Besides, electric current required to drive a motor sees its peak just at the beginning of rotation. Therefore, a surge of power would be the result if such driving devices started their operations in unison.
Furthermore, even in the case that only one color, for example, monochrome (usually, black) print is required, other image-forming unitsxe2x80x94units for cyan, magenta, and yellowxe2x80x94also have to work with an xe2x80x9cidlexe2x80x9d printing motion. This wasteful motion produces friction between the members forming the photosensitive drum, the developing roller, and the cleaner, thereby shortening their useful life.
In the event of an interruption of the printing due to paper jamming or other malfunctions, the toner image left of the drum and the inter-stage transfer belt has to be cleaned out for the next printing. Being typical of the structure of the tandem type apparatus, the transfer belt requires a much longer time to travel one rotation than the drum needs to rotate one turn. That is, the drum unnecessarily has to have several turns while the belt travels at least one rotation. This structural inconvenience also causes friction between parts forming the image-forming units, thereby reducing their longevity.
The object of the present invention is to provide a color-image forming apparatus having a structure that minimizes damage in the photosensitive drum and the inter-stage transfer belt due to friction occurring between them.
The apparatus of the present invention includes an exposure unit, plural image-forming units, and an endless inter-stage transfer belt.
The image-forming units are organized in a tandem type arrangement, and each contains: i) it""s own photosensitive drum on which an electrostatic latent image is formed by the exposure unit; and ii) it""s own developing unit for forming a toner image by applying toner onto the latent image. In addition, each unit has toner of a different color. Looped over plural rollers in its traveling path, the endless inter-stage transfer belt rotates to run along the direction of the arrangement of the image-forming units. A full-color toner image is formed on the belt as it runs, thereby overlapping the toner images formed on the drums upon another.
In the process, the belt comes into contact with the drum only while the toner image is transferred from the drum onto the belt. Keeping the belt from contact with the drum except for during the transfer process of the toner image reduces damage or abrasion from friction between the drum and the belt at the start of rotation. That is, the image-forming unit and the transfer belt have a longer service life.